French patents FR 2,577,564, FR 2,577,168, FR 2,415,000, FR 2,436,676 and FR 2,659,085 describe coextrudable polyvinylidene fluoride (PVDF) compositions which make it possible to make it adhere to thermoplastic resins with which it is not compatible. The complexes are obtained by coextrusion.
In this manner, it is possible, for example, to coextrude PVDF with the composition intended to make it adhere to the PVDF-incompatible polymer, and then to apply this complex to the incompatible polymer or alternatively to directly coextrude the three layers: PVDF/composition necessary for adhesion/PVDF-incompatible polymer.
Such complexes are advantageous insofar as their structure is homogeneous or integral, in contrast with heterogeneous complexes such as, for example, multilayer materials in which the layers are adhesively bonded together and in which the interfaces are not interpenetrated. Within the context of these inventions, which form the subject of the abovementioned patents, mutual interdiffusions of the polymers are demonstrated, thereby leading to a structure which is not discontinuous (as in the bonded assembly), but to a continuity within the material which will be referred to as an integral complex.
In a system such as that described in the patents mentioned above, the PVDF is in contact with an adhesion binder with which it is coextruded. Once applied to the PVDF-incompatible polymer, either by coextrusion or by hot-pressing, this binder can ensure adhesion between PVDF and incompatible polymer only insofar as it is not degraded during the use of the complex. There are many reasons for degradation:hydrolysis, attack by solvents or other chemical products, degradation by photo-oxidation.
The subject of the present invention thus relates to an integral complex containing at least three layers, which is intended to adhere to a PVDF-incompatible polymer. A second subject of the invention relates to the three-layer complexes coextruded directly with the PVDF-incompatible polymer or polymers. The invention also relates to the process for the manufacture of the three-layer integral complex, this three-layer complex being declared integral as opposed to the same three layers which would have been adhesively bonded and which do not have this continuous structure.
More precisely, the invention firstly relates to an integral complex comprising at least three layers consisting respectively of PVDF, of PVDF rendered opaque to UV and visible radiation, and of a composition which allows adhesion of the PVDF to a PVDF-incompatible thermoplastic polymer.
This complex is capable of protecting the adhesion composition from external attacks.
The expression external attacks is understood to denote in particular the actions of water, chemical products, solvents, gases and radiation in the UV and visible ranges.
The integral and homogeneous complexes forming the subject of the invention have improved longevity with respect to chemical and atmospheric agents, coupled with surface properties which are superior to those of all the other existing complexes.
It is easy to understand that, in the processes mentioned above, a pure PVDF at the surface was incapable by itself of providing the adhesion binder with UV protection, since it is itself UV-permeable. Although, admittedly, it is possible to incorporate organic anti-UV agents into the PVDF, these agents, which are incompatible with the PVDF matrix, migrate to the surface and disappear rapidly, by leaching with water for example, and, in any case, cease to function themselves after a certain period of time. Another way of proceeding consists in incorporating fillers and pigments which act as a UV barrier. It is observed in this case that the essential properties of the PVDF are sacrificed, and in particular its impermeability to liquids and gases. This loss of impermeability is, in this case, the cause of attack of the adhesion binder, and the adhesion is seen to disappear due to hydrolysis and oxidation, if not by solvent attack.
Moreover, the presence of fillers and pigments in the PVDF leads to a microporosity of the latter, which removes the non-stainability, anti-soiling and anti-graffiti qualities therefrom.
Furthermore, this permeability has the effect of allowing various products added to the adhesion binder or to the incompatible substrate to migrate to the surface of the PVDF layer, these products and the degradation products thereof forming a screen between the external agents (UV, O2, H2O, etc.) and the surface of the PVDF.
All of these drawbacks are eliminated in the three-layer integral complex which forms the subject of the invention.
This complex is generally intended to form, with a PVDF-incompatible material, a novel 4-layer integral complex, either by coextrusion or by hot-pressing.
Among the PVDF-incompatible polymers which may be mentioned in particular are the chloro vinyl polymers such as the polyvinyl chloride or the polyvinylidene chloride, the styrene polymers such as the polystyrene or impact polystyrene, the polycarbonates, the polyurethanes, the grafted styrene-acrylonitrile-acrylic elastomers copolymers, the acrylonitrile-butadiene-styrene copolymers, the polyamides, the polyether (block) amides, the polyolefins such as polyethylene or polypropylene or copolymers thereof, the polyphenylene ethers (PPE) and particularly their alloys with polystyrene and/or polyamide. The expression PVDF-incompatible polymer is also understood to refer to mixtures of these polymers, as well as these polymers or mixtures containing fillers, plasticizers or stabilizers.
It goes without saying that the layer of incompatible polymer may itself be associated in any manner with another polymer, a metal and/or a fabric.
As PVDF which may be used in the invention, it will be preferred to use a PVDF homopolymer, but also included under this name are copolymers containing at least 70% by weight of residues of vinylidene fluoride molecules and mixtures thereof with the homopolymer, as well as mixtures of PVDF and/or PVDF copolymer with poly(C114 C4)alkyl methacrylate and/or poly(C1-C4)alkyl ethylacrylate containing at least 60% by weight of PVDF homo- or copolymer.
For the requirements of the invention, the three-layer integral complex will include at the surface a PVDF layer as defined above, containing not more than 5% by weight of fillers or pigments or other additive. Preferably, this will be a pure PVDF homopolymer, without additives, pigments or fillers.
This layer will have a light transmission such that the absorption of photons of wavelength 800 nm, measured by spectrophotometry equipped with an integration sphere on a film 50 xcexcm in thickness, is preferably less than 70%.
Fillers, pigments, UV-absorbing agents or UV-stabilizing agents, plasticizers, etc. necessary to obtain the opacity to UV and visible rays will be added to the second layer of PVDF. This second layer will have a light transmission such that the absorption of photons of wavelengths between 200 and 800 nm, measured by spectrophotometry equipped with an integration sphere on a film 50 xcexcm in thickness, is preferably greater than 90%.
By way of illustration of additives which render the PVDF opaque to UV and visible rays, mention will be made in particular of titanium oxide, zinc oxide, carbon black, antimony oxide, inorganic or organic pigments, and benzophenone-based UV-absorbing agents.
By way of illustration of adhesion compositions of the third layer, reference may be made to the compositions described in the five patents mentioned above, which are incorporated here by way of reference.
Among these compositions, mention may be made in particular of compositions based on polyalkyl methacrylate, on polyurethane or alternatively on ethylene/CO polymer.
The respective thicknesses of these three layers will preferably be:
1st layer between 10 xcexcm and 300 xcexcm,
2nd layer between 10 xcexcm and 300 xcexcm,
3rd layer between 10 xcexcm and 200 xcexcm.
It will be possible for these three layers to be coextruded according to the usual techniques for the coextrusion of films, sheets and plates (as described in the patents mentioned).
It goes without saying that the subject of the invention is not limited to the coextrusion of this three-layer integral complex, but that it is extended to the one-stage coextrusion of this triple layer with the PVDF-incompatible polymer, as well as to the complexes which the hot-pressed three-layer complex forms with the PVDF-incompatible polymer, according to the standard techniques of coextrusion and of hot bonding.
The invention relates also to the complexes formed by the three-layer complex bound or hot-pressed with metallic substrates such as steel or aluminium, wood-based substrates and/or fabrics.
Lastly, also forming part of the invention is the production of a PVDF-incompatible polymer complex in which at least one face is protected by at least the three-layer integral complex, it being possible for the others to be protected by, besides this triple layer, a double layer which forms the subject of the patents mentioned above, or by another material which protects the PVDF-incompatible polymer.
The examples which follow illustrate the invention.